Looking above the waist, dataviz style

February 12, 2018
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(This article was originally published at Junk Charts, and syndicated at StatsBlogs.)

I came across this chart on NYU's twitter feed. 

Nyu_admissions

Growth has indeed been impressive; the dataviz less so. Here's the problem with not starting the vertical scale of a column chart at zero:

Redo_nyuapplications_0

In a column chart, the heights of the columns should be proportional to the data. Here they are misaligned because an equal amount has been chopped off below 30,000 from all columns. The light purple that I layered on top of the chart presents the correct heights of the columns, assuming that the first column for 2007 indeed properly encoded the data.

The dark purple top of each column represents the "lie factor." It is the amount of exaggeration created by chopping off those legs. The lie factor is of Ed Tufte coinage.

***

The designer probably wanted to show the year-to-year trend more starkly. Doubling the number of applications in 10 years is pretty impressive. The solution is not to chop off the legs but to look above the waist. You can't fix the column chart but you can switch to a line chart, as follows:

Redo_nyuapplications

In a line chart, we are mostly concerned with the changing slope of the line segments going from year to year. The slopes encode the year-on-year growth rates. 

 



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